Active RFID trilateration for indoor positioning

This paper addresses the investigation of RFID trilateration for indoor positioning. At first some background of RFID was introduced. On this basis it was deduced how the measured signal strength can be converted into a distance. The converted distances could be used for determining the position of the reader by means of trilateration. Finally, the approach was tested in a test environment near the Vienna University of Technology.
For the signal strength to distance conversion a polynomial model was employed. Signal strength measurements were performed on baselines with a point interval of 2 m. The resulting mean value of the residuals in the conversion of the signal strength into a distance is nearly zero (i.e., below ± 2*10-11 m) using a polynomial model with an order of p = 3. These polynomial models were then used for the deduction of ranges to the tags for the location of points in static and kinematic tests. In the static test the test points were located with an accuracy of better than ± 0.35 m in X-direction and ± 1.44 m in Y-direction. In the kinematic tests lower accuracies in the range of ± 2 to 3 m were achieved, but the trend of the moving user was always determined correctly. The achieved positioning accuracies have proven that RFID trilateration can be successfully employed in a small range to the tags.
In the future, we will perform more experiments to improve the polynomial models for signal strength to distance conversion. At the same time the algorithms will be tested in a larger and complicated environment and with more tags.

Acknowledgements
This research is supported by the research project P19210-N15 “Ubiquitous Carthograpy for Pedestrian Navigation (UCPNAVI)” founded by the Austrian Science Fund (Fonds zur Förderung wissenschaftlicher Forschung FWF).